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NMR Studies on Li+, Na+ and K+ Complexes of Orthoester Cryptand O-Me2-1.1.1

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NMR Studies on Li+, Na+ and K+ Complexes of Orthoester Cryptand O-Me2-1.1.1 Int. J. Mol. Sci. 2015, 16, 20641-20656; doi:10.3390/ijms160920641 OPEN ACCESS International Journal of Molecular Sciences ISSN 1422-0067 www.mdpi.com/journal/ijms Article NMR Studies on Li+, Na+ and K+ Complexes of Orthoester Cryptand o-Me2-1.1.1 René-Chris Brachvogel, Harald Maid and Max von Delius * Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Henkestr. 42, 91054 Erlangen, Germany; E-Mails: [email protected] (R.-C.B.); [email protected] (H.M.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +49-9131-85-22946; Fax: +49-9131-85-26864. Academic Editor: John Hardy Received: 10 July 2015 / Accepted: 24 August 2015 / Published: 31 August 2015 Abstract: Cryptands, a class of three-dimensional macrobicyclic hosts ideally suited for accommodating small guest ions, have played an important role in the early development of supramolecular chemistry. In contrast to related two-dimensional crown ethers, cryptands have so far only found limited applications, owing in large part to their relatively inefficient multistep synthesis. We have recently described a convenient one-pot, template synthesis of cryptands based on O,O,O-orthoesters acting as bridgeheads. Here we report variable-temperature, 1H-1D EXSY and titration NMR studies on lithium, sodium, and potassium complexes of one such cryptand (o-Me2-1.1.1). Our results indicate that lithium and sodium ions fit into the central cavity of the cryptand, resulting in a comparably high binding affinity and slow exchange with the bulk. The potassium ion binds instead in an exo fashion, resulting in relatively weak binding, associated with fast exchange kinetics. Collectively, these results indicate that orthoester cryptands such as o-Me2-1.1.1 exhibit thermodynamic and kinetic properties in between those typically found for classical crown ethers and cryptands and that future efforts should be directed towards increasing the binding constants. Keywords: supramolecular chemistry; cryptands; orthoesters; dynamic covalent chemistry; NMR spectroscopy; host-guest complexes Int. J. Mol. Sci. 2015, 16 20642 1. Introduction Crown ethers (macrocyclic oligomers of ethylene glycol) [1,2] and cryptands (bicyclic structures made up of oligoethylene glycol arms and trialkylamine bridgeheads) [3–6] are iconic supramolecular hosts that through their interaction with small guest ions have contributed significantly to our current understanding of non-covalent interactions. The archetypal guests complexed by these hosts are alkali metal ions and the kinetics, as well as the thermodynamics of this association process, have been comprehensively studied and reviewed [7]. Prior to 2015, only few studies reported significant variations to the crucial bridgehead architecture of cryptands. For example, Coxon and Stoddart have described the multi-step synthesis (total yield less than 1%) of a 1.1.1-tris(hydroxymethyl)ethane- capped cryptand that exclusively possesses oxygen donor atoms [8]. Parsons and coworkers have synthesized related, yet less symmetric, macrobicycles in which two glycerol motifs act as bridgeheads [9–11]. Saalfrank and coworkers have self-assembled metallocryptates in which iron ions act as bridgeheads and a mix of nitrogen and oxygen donors is available for cation binding [12,13]. Lehn and Nelson have prepared tripodal imine-based bimetallic cryptates, featuring mainly nitrogen donors [14–17] and Voloshin, as well as others have reported studies on kinetically-inert “clathrochelates”, in which three glyoxime-type ligands are capped by borate bridgeheads [18–21]. We have recently described a one-pot, template synthesis of monometallic cryptates based on O,O,O-orthoester bridgeheads [22]. As shown in Figure 1, both the unique structural (tripodal geometry) [23] and dynamic (acid-catalyzed exchange with alcohols) [24] features of orthoesters are responsible for the remarkable efficiency of this self-assembly process. Due to the fact that this new class of cryptands is constitutionally dynamic in the presence of acid and the cage structure can be disintegrated at low pH, we anticipate that rather unique curiosity-driven (subcomponent self-sorting and systems chemistry), as well as application-oriented (controlled guest release and drug delivery), studies can be pursued with this new class of compounds. However, to engage in such endeavors, a detailed knowledge of the properties of these compounds and their accommodation of guest ions is needed. In our initial communication on the sodium-templated self-assembly of orthoester cryptates [22], we did only report preliminary data on the thermodynamics and kinetics of the binding of different metal ions with orthoester cryptand o-Me2-1.1.1. Herein, we report more comprehensive physicochemical data and we discuss the implications of our findings on future research directions. 2. Results and Discussion Four compounds had to be prepared to carry out the NMR studies described herein: o-Me2-1.1.1, + − + − + − [Na ⊂o-Me2-1.1.1]BArF , [Li ⊂o-Me2-1.1.1]TPFPB and [K •o-Me2-1.1.1]BArF . As shown in + − Figure 1, cryptate [Na ⊂o-Me2-1.1.1]BArF was prepared in 67% yield using the templated self-assembly reaction (reaction scale typically 50 mg in respect to isolated product). Cryptand o-Me2-1.1.1 was + − obtained by treating a chloroform solution of [Na ⊂o-Me2-1.1.1]BArF with anion exchange resin Lewatite® MP-64, which led to the precipitation of NaCl, along with a solution of the desired empty cage (reaction scale typically 10 mg in respect to product; due to its high sensitivity to acid-catalyzed hydrolysis, cryptand o-Me2-1.1.1 is best freshly prepared). The lithium and potassium complexes + − + − [Li ⊂o-Me2-1.1.1]TPFPB and [K •o-Me2-1.1.1]BArF , respectively, were obtained by titration of Int. J. Mol. Sci. 2015, 16 20643 the corresponding metal salts to cryptand o-Me2-1.1.1. For details regarding these syntheses, please refer to the experimental section. Figure 1. Synthesis route for the different alkali metal complexes. (i) 5% TFA, MS 4 Å, CDCl3 (10 mM in respect to NaBArF (sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)), ® RT; (ii) Lewatit MP-64 anion exchange resin, CDCl3, 6h, RT; (iii) 1.0 equiv. LiTPFPB (lithium tetrakis(pentafluorophenyl) borate ethyl etherate), CDCl3, RT; (iv) 1.0 equiv. KBArF (potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), CDCl3, RT. Figure 2 gives an overview of the 1H NMR spectra of the different investigated alkali metal complexes in chloroform. The “empty” cryptand o-Me2-1.1.1 exhibits a singlet at 1.43 ppm and a multiplet at 3.73 ppm (Figure 2a). Fundamental differences were observed in the spectra that were obtained when 0.5 equivalents of different metal salts (NaBArF, LiTPFPB, KBArF) were added to o-Me2-1.1.1 via titration. For the sodium and lithium complexes (Figure 2b,c), we observed NMR spectra indicative for slow cation exchange (different sets of signals for the cryptates and for the cryptand), whereas fast exchange was observed for the potassium complex (one average signal set, see Figure 2d). These results indicate at a qualitative level that the kinetics for the process of one metal ion hopping from one orthoester cage into another are significantly slower for lithium and sodium ions than for potassium ions. To investigate whether this seemingly drastic difference in the kinetics of cation exchange is also reflected in the thermodynamic binding strength, we performed 1H NMR titrations in solvent acetonitrile. This solvent was chosen because all studied metal salts were soluble therein and because it facilitated fast cation exchange on the NMR timescale, so that binding isotherms could be obtained. Using 1D EXSY [25] (exchange spectroscopy) and VT (variable temperature) NMR spectroscopy we also studied the kinetics of cation exchange at a quantitative level. These measurements are described for each alkali metal individually in Sections 2.1 to 2.3, while Section 2.4 discusses the effect of different counter-anions, and Section 2.5 will provide an overview and a discussion of the results. Int. J. Mol. Sci. 2015, 16 20644 1 Figure 2. Partial H NMR (400 MHz, 298 K, CDCl3) stack plot; (a) only o-Me2-1.1.1; + − (b) 1:1 mixture of o-Me2-1.1.1 and [Na ⊂o-Me2-1.1.1]BArF ; (c) 1:1 mixture of o-Me2-1.1.1 + − + and [Li ⊂o-Me2-1.1.1]TPFPB ; and (d) 1:1 mixture of o-Me2-1.1.1 and [K •o-Me2-1.1.1] BArF−. *: Peaks in grey color correspond to water. + − 2.1. Thermodynamic and Kinetic Properties of [Na ⊂o-Me2-1.1.1]BArF In previously-published NMR titration experiments [22], we treated empty cryptand o-Me2-1.1.1 with NaBArF in solvent CDCl3 (saturated with D2O). Under these conditions, we observed slow exchange on the NMR time scale and we could show by competition experiments with classic cryptand [2.2.1] and crown ether 15-crown-5 that the binding constant for this orthoester cryptand lies in between the binding constants for the two competing classic macro(bi)cyclic hosts. To obtain more meaningful thermodynamic information on metal binding, we proceeded to titrate a solution of NaBArF (0 to 1000 mol %) to o-Me2-1.1.1 in solvent CD3CN (Figure 3a). In this case, fast exchange on the NMR time scale allowed a quantitative analysis of the system’s thermodynamics. By fitting the data of the binding isotherm, using program HypNMR [25], we were able to determine the binding constant of the complex. Interestingly, an excellent fit of the data could only be obtained when both a 1:1 and a 1:2 complex was taken into account (“1:2” indicating one metal and two −1 cryptands). As shown in Figure 3b, the resulting association constants (KA, Na) in CD3CN are 1330 M −1 (K1) for the formation of the 1:1 complex and 10 M (K2) for the equilibrium between the 1:1 and the 1:2 complex [26].
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